I have a feeling I already know the answer to this (that it's a failing battery) but would love some bits of advice before I go and buy a new one. I have searched and found hit and miss answers so far.

A few months back, the car speedometer started playing up. It would drop to 0 and stay there, sometimes anywhere up to 30 miles. When she took it for the MOT, she left a note asking the mechanic to have a look at the speedo and he said he had done so and it was all good now. To be fair, it's been fine for a while. Then a few weeks ago, it started doing it again.

Now it drops to 0, and a few seconds later, the airbag light usually comes on and the little screen reads AIRBAG FAULT.

In the last few days, it's also started randomly cutting out, when these things happen. The steering lock doesn't come on or anything like that, but obviously it's not nice when it happens.

I have gone out on a limb and assumed it's the battery and I've promptly ordered a little plugin voltmeter (always wanted a reason to buy one) and I am hoping it'll point me in the right direction. My problem is that the battery is a relatively new looking Exide. I've only had good experiences with Exide as well. Is there anything else it could be? Before I go and buy a new battery at a fairly sizeable cost.

It could be the alternator not functioning very well, but your voltmeter should tell you that as well. With the engine running, you should see at least 13.4V, and with it stopped 12.6V.

If the car is not heavily used, and then only for short runs, the battery can become "sulphated". Provided the condition is not too severe, the battery can be rescued with a suitable charger (e.g. this), which you can thereafter employ every couple of weeks, to prevent a recurrence.